“There’s a shooter at the Edgartown school,” Edgartown police officer David Rossi shouted. “There’s gunshots! You gotta get somebody to the Edgartown school quick!”

Officer Rossi stood outside the back entrance of the school cafeteria. His urgent 911 call was the trigger for an Island-wide response to an armed intruder as part of a Saturday morning drill involving every Island police and emergency services department.

The call was made at 9:02 AM. By 9:05 AM, Edgartown police officers wielding unloaded AR-15 assault rifles were leaping from their cruisers at the school grounds. Soon after, officers from other Island towns arrived. Off duty police officers also responded from their homes.

Police established a perimeter. Ambulances from several towns arrived, and a triage area was set up in the school gym. At the same time, members of the Martha’s Vineyard tactical response team swept the building.

Since September, Island fire, emergency medical services (EMS) and police from all towns, along with Edgartown school administrators, have met monthly to plan for an “active emergency.” The updated protocol followed a series of school shootings nationwide over the past few years. A state police helicopter that was planned to appear was unable to participate due to fog concerns.

Only Edgartown police officer Ryan Ruley and deputy fire chief and ambulance coordinator Alex Schaeffer knew what would happen before the drill, a strategy emergency services uses to simulate a realistic situation.

“Everything was based off what the officers found,” Mr. Schaeffer told the Times following the drill. “The plan that we’ve been working on went well — that framework; there’s parts you can fix, to make it perfect, but it was good. Communication continues to be an issue on the Island, and drills like this show that necessity.”

To add an element of surprise, Mr. Schaeffer and Mr. Ruley added a second shooter to the drill scenario. Although the responding officers were aware of one shooter, they did not know about the second shooter, who withheld fire until the first police arrived.

Edgartown police officer and firearms expert Joel DeRoche, who serves as the school resource officer two days each week, played the role of the second shooter.

“In my 18 years, I’ve never attended any training that’s come this close to reality,” Mr. DeRoche told The Times at the conclusion of the drill. “The school resource officer was in the cafeteria, because it’s a heavily populated area and a target. I was threat number two, on the second floor, and became active once the first shooter was taken care of. At first, as the school resource officer, you’re it. The cavalry took three minutes. That’s a long three minutes. That’s three hours.”

He said the main problem highlighted by the drill were radio issues. “Communication, communication, communication; that was very difficult for a variety of reasons,” he said. “Nothing we can’t figure out though, sooner rather than later.”

“Today was a baseline, now we’ll see how to tweak it. It’s a fluid plan, there’s nothing we won’t consider,” he said. “This wasn’t a bunch of cops playing war games, it’s so important for us to be prepared.”

He thanked Edgartown school principal John Stevens. “Without his gracious hospitality we couldn’t have done this,” officer DeRoche said. “We appreciate his proactive approach to school security.”

Edgartown assistant principal Anne Fligor, who was part of the monthly planning meetings, was working in her office when the drill began. She announced the lockdown over the school intercom, which on a regular school day would notify teachers to lock classroom doors and gather students in a corner of the room until police cleared the building. On this day, only a few faculty members and cafeteria staff were in the building, as well as several volunteer victims and two EMS dummies.

“I was doing actual work, a spreadsheet for how much time is allotted to each subject,” Ms. Fligor said. “I’m incredibly glad we did the drill, just to see what works and what we need to work on. It’ll be interesting to hear from everyone how it went, and if we need to alter the emergency plan.”

Edgartown school principal John Stevens noted the cooperation of parents. “The teachers and community were well informed and there was no pushback,” he said. “There was a lot of support, actually. It’s good to see that people put a value on school security.”

A major factor was cooperation between different departments and different plans. “Island-wide cooperation is what it’s all about,” said school superintendent Jim Weiss.